30 TOPICS IN CLINICAL CHIROPRACTIC/VOLUME 9, ISSUE 3, 2002

Top Clin Chiropr 2002; 9(3): 30–43Source: Adapted with permission from the forthcoming book, Principlesand Practice in Chiropractic, ed. 3, Scott Haldeman, ed., © 2003, TheMcGraw-Hill Companies.30

J.M. Menke, Integrative Healthcare Consulting, 1836 Bryant Street, PaloAlto, CA 94301.

Activator Methods Chiropractic Technique

A

Purpose: A review of the Activator Methods chiropractic technique (AMCT) and Activator adjustinginstrument (AAI) is presented. History and development of the technique and its evidence basis and safetyissues are discussed. Method: Activator history and associated body of research literature on both thetechnique and the instrument are critically reviewed. Included are basic science research in biomechanics,neurophysiology, and clinical research in AMCT analysis reliability, case studies, prospective cohorts,randomized group clinical outcomes, and comparisons to hands-only treatments. Summary: AMCT and theAAI represent a system and mode of delivery based on rational and empirical evidence that continues to beinformed by an active and growing body of clinical research. In comparison to other techniques, AMCTappears equivalent to various hands-on approaches, but with less physical demand on the doctor ofchiropractic. As with all spinal manipulative and other therapies, risks of adverse effects exist, but appear tobe minimized due to force and velocity characteristics of the AAI. Needs for future research directions are alsodiscussed. Key words: chiropractic, clinical protocol, leg length inequality, research

Arlan W. Fuhr, DCPresidentActivator Methods InternationalPresidentNational Institute of Chiropractic ResearchPhoenix, Arizona

J. Michael Menke, MA, DCIntegrative Healthcare ConsultantPalo Alto, CaliforniaPostgraduate FacultyIntegrative ChiropracticNational University of Health SciencesLombard, Illinois

CTIVATOR METHODS Chiropractic Technique(AMCT) is based on a dynamic approach to spinal and

somatic dysfunction in which biomechanical lesions are isolatedand identified through patient movements designed to isolateand reveal the exact areas of dysfunction. With detection,treatment is provided through a mechanical instrument designedto deliver a preset amount of force precisely to correct the lesion.The primary indicator of lesion and response to treatment is thedifference in leg lengths of the prone patient.

AMCT is taught in the majority of U.S. chiropractic col-leges and is offered at international chiropractic colleges.Approximately 45,000 doctors of chiropractic throughout theworld now use this technique. Recent surveys by the NationalBoard of Chiropractic Examiners show that AMCT is one ofthe two most widely used chiropractic techniques in theUnited States.1 In the United States, the percentage of practi-tioners using AMCT increased from 51.2% in 1991 to 62.8%in 1998. Chiropractors reported using AMCT with 21.7% oftheir patients in 1998. In Europe, it was estimated the tech-nique was used in 14% of chiropractic cases in 1994.2 TheAMCT is also widely used in Canada3,4 and Australia.

Activator cofounder and president, Arlan W. Fuhr, DC,initiated scholarly research for the purposes of improving anddemonstrating AMCT’s value to chiropractic, the broaderhealth care system and the general public. Research anddevelopment of AMCT has yielded returns in the knowledgeof anatomy, biomechanics, and neurophysiology. Many of

Fuhr and Menke: Activator Methods’ Chiropractic Technique 31

these studies addressed fundamental questions not fully un-derstood and articulated by the chiropractic profession, in-cluding spinal biomechanics and energy and resonance char-acteristics of the chiropractic adjustment.

HISTORY OF ACTIVATOR METHODS

Instrument adjusting in chiropractic dates to the early yearsof the profession. According to Keating, instrument adjusting“was apparently an ancient and indigenous American art.”5 Hecites a report by Dorn,6 a chiropractor who bought an instru-ment that may have been used by Montana Crow Indianmedicine men to relieve digestive ailments. The Crow used atype of thrust perhaps similar to a percussion treatment. Asearly as 1901 Minnesota chiropractors were using the “stickmethod” developed by Thomas H. Storey, DC, a 1901 gradu-ate of D.D. Palmer.7 Storey employed a wooden mallet and astick covered with what appeared to be a rubber tip from acrutch to “set the spine.”8 D.D. Palmer himself, sometimeafter 1910, had a “rubber hammer (pleximeter) with which heexperimented adjusting vertebrae.”9

During the 1960s, Drs. Warren C. Lee and Arlan W. Fuhrpracticed Logan basic and Toftness low-force adjustmenttechniques. Lee and Fuhr sought to establish better, moresensitive clinical assessment methods for identifying subtlemusculoskeletal dysfunction and segmental facilitation and toassess when improvement in the patient’s condition hadoccurred. They experimented with procedures to assesspreadjustment status and postadjustment clinical outcomes.In 1965, Fuhr learned the directional nonforce technique.These procedures involved preadjustment and postadjustmentleg length measurements and double thumb-lock toggle ad-justing. Fuhr later met Mabel Derifield, DC, and integrated theDerifield system of pelvic analysis using a relative leg lengthmeasurement.

In 1985, Activator Methods was awarded a small businessinnovative research National Institutes of Health (NIH) Grantfor the study of safety and effectiveness of the mechanicaladjusting instrument on osseous structures. In 1990, the AMCTpublished the critical self-review entitled “The Current Statusof Activator Methods Chiropractic Technique, theory, andtraining.”10 The first peer-reviewed paper concerning AMCThistory appeared in the Chiropractic Journal of Australia in1994,11 and a more complete history subsequently appeared inthe 1997 AMCT textbook. Cooperstein12 noted AMCT to bethe first and perhaps only technique system to apply theKaminski model for chiropractic technique validation.13

DEVELOPMENT OF AMCT ISOLATION TESTS

Dr. Fuhr discovered isolation tests circa 1976, when henoted leg length changes in response to positional changes of

body structures affected by somatic dysfunction. Fuhr ob-served and codified observations of musculoskeletal imbal-ances in the lumbar, cervical, and the extraspinal regions ofthe body. This became the system of analysis known today asAMCT isolation testing.

Prone leg length comparisons in response to isolation testingbecame the AMCT indicator for determining sites and occasionsto adjust. The functional short leg was named the pelvic deficientside by Fuhr. The functional short leg is not anatomically short,but rather appears shorter because of presumed obliquity in thepelvis. This phenomenon led to the development of a series ofprovocative maneuvers to test for joint mechanical dysfunc-tions of the vertebrae and other articulations. As the techniquewas taught to other chiropractors, these chiropractors in turnbegan sending in their own clinical observations. These weretested by Lee and Fuhr and eventually submitted to a standardof care review panel of AMCT instructors. Tests consideredclinically useful were incorporated into the main body ofAMCT technique protocol.

RELIABILITY OF LEG LENGTH ANALYSIS

Reliability studies on prone leg length inequality (ActivatorMethods Position 1) established it as a potentially stableclinical phenomenon, with a good degree of agreement amongtrained examiners.14–17 Jansen and Cooperstein18 found thatthe functional short leg is a reproducible clinical entity whenthey used a chiropractic table with friction-free plates. Theyobserved leg length changes under a variety of postures andmovements in prone subjects.

Four examiners who evaluated 30 subjects by means of theAMCT leg length procedure in Position 1, found that five ofsix pair-wise interexaminer comparisons yielded “good” con-cordance (g = 0.53).14 Order of examiners was not random-ized, creating the possibility of unwanted order effects. Theirresults were confirmed in a later study by Nguyen HT andcolleagues, in which 34 subjects were examined by twoexperienced AMCT instructors, yielding a total agreement of85% (g = 0.66, p < 0.001).17 In this study the order ofexaminers was randomized to control for order and practiceeffects.

Youngquist, Fuhr, and Osterbauer16 reported the first studyto evaluate the interexaminer reliability of isolation testing.They sought to determine whether prone leg length analysis inassociation with an isolation test maneuver was reproducible.Two examiners evaluated 72 subjects who were divided intogroups of 34 and 38 patients on two separate occasions.Subjects were evaluated for the presence or absence of atlassegment (C1) subluxation by observing leg length inequality.Agreement was well beyond chance for the two samples, withg = 0.52 (p < 0.01, n = 24) for the first sample and g = 0.55(p < 0.001, n = 48) for the second.

32 TOPICS IN CLINICAL CHIROPRACTIC/VOLUME 9, ISSUE 3, 2002

De Witt and colleagues15 investigated leg length inequalityfollowing isolation maneuvers. They employed sophisticatedoptoelectric measuring equipment. Eight healthy subjectswere compared to eight subjects with a history of chronicspinal complaints. As a result of prone neck extension (“headup”) maneuvers (the C5 isolation test), patients exhibitedsignificantly more asymmetrical (right versus left) heel move-ment than controls.

The reliability of leg length checks is distinct from validityof leg length observations. The former refers to the consis-tency of findings (over time or between examiners), the latterto the clinical meaningfulness of observed leg length inequal-ity. Although studies of the reliability of these procedureshave provided support for consistent findings among examin-ers (in Position 1), the validity of AMCT leg length analysis(and associated isolation, stress and pressure tests) has notbeen directly studied to date.

However, several investigations provide indirect data bear-ing on the meaningfulness of AMCT leg length evaluations.As mentioned previously, DeWitt15 showed isolation maneu-vers (cervical extension) produced measurable leg lengthchanges. Shambaugh and colleagues19 demonstrated that rightand left cervical rotation produced leg length alterations.

AMCT BASIC SCAN PROTOCOL

The AMCT basic scan protocol is based on the premise thataddressing the most common sites of involvement will resolvemost minor dysfunctions or subluxations and of vertebrae notdirectly included in the basic scan. Judicious use of the basicscan protocol is consistent with a sound tradition in chiroprac-tic clinical practice (i.e., avoidance of overadjusting). Oneway to limit excessive adjusting is to adjust only majorsubluxations occurring at the major stress and transitionpoints in the spine, typically the transitional vertebra areas.Subluxations of other vertebrae may occur secondarily or ascompensations to these primary stresses in the dynamic struc-ture of the spine by fixation or aberrant motion. Adjustment ofmajor segmental dysfunctions may also eliminate compensa-tory or minor subluxations.20 Tables 1 and 2 outline theprincipal assessments of the AMCT basic scan protocol.Position 1 refers to the patient prone with knees extended.Position 2 is the patient prone with knees passively flexed bythe chiropractor.

The vertebral motor units and other articulations includedin the AMCT basic scan protocol are listed in Table 3. Notethat there are only a few “key” vertebrae listed for each regionof the spine.

DEVELOPING THE ACTIVATOR ADJUSTINGINSTRUMENT

The Activator adjusting instrument (AAI) was developedto mechanically replace the need for thumb toggling, a tech-nique that produced extreme fatigue, muscle strain, and fre-quent elbow injury on chiropractors who practiced it. Theideal mechanism needed to produce a thrust, reduce physicalstress on the clinician, and precisely control the speed, force,and direction of the adjustive thrusts.21

In 1966, Lee and Fuhr initially tried a dental impactor,designed to tap amalgam into teeth. However, this instrumentfailed as a spinal adjusting device because the impactor did nothave enough speed or force. Several other devices were testedincluding a center punch (which required too much preloadpressure) and an instrument with a moving stylus that causedtoo much patient discomfort. In 1967, Fuhr tried a surgicalimpact mallet designed to split impacted wisdom teeth. Thescalpel was replaced with a brake shoe rivet, and a smallrubber doorstop was attached to the end. This device used ahammer striking an anvil to produce a sudden shock forcemuch like a croquet mallet striking two croquet balls. Thisapparatus tested successfully on patients and was the firstfunctional ancestor of the modern AAI. This instrument wasmodified and used until 1976.

A later version that held up to the demands of daily practicewas eventually designed by Dr. Freddy Hunziker. The Activa-tor I was designed and built a more reliable internal mecha-nism for the instrument, which used a hammer-anvil effect toproduce a safe, reliable, and controlled force to adjust osseousspinal structures.11 The adjusting instrument was subsequentlynamed the Activator and the analysis became known as theActivator Methods Chiropractic Technique.

RESEARCH AND DEVELOPMENT OF THE AAI

AMCT is one of the more extensively studied techniquesystems in chiropractic. The AAI, listed under the category of“mechanical force, manually assisted procedure,” yielded a“promising to established” rating at the 1992 Mercy Centerclinical guidelines consensus conference.22 A consensus panelcommissioned by the Canadian Chiropractic Associationgranted a similar rating for the AAI.23

Biomechanical research

Greater understanding of biomechanics and neurophysi-ological research has led to refinements in the AAI. In the1980s and 1990s, research on spinal manipulation shed light

Fuhr and Menke: Activator Methods’ Chiropractic Technique 33

on the nature of the chiropractic adjustment in terms of time,force, duration and physiological effects—as delivered byhands only and by the AAI. There have been two generalcategories of investigation: biomechanical and neurophysi-

Contact medial knee joint—LOD is lateral and inferior

Contact medial border—LOD is posterior-superior andlateral

Contact lateral knee joint—LOD is medial-inferior

Contact inferior lateral aspect—LOD is posterior-superior and medial

Step 1: Contact base of sacrum on side opposite PD—LOD is anterior-inferior

Step 2: Contact crest of ilium—LOD is inferior-medial

Step 3: Contact ischial tuberosity—LOD is anterior-inferior

Step 1: Contact spine of ischium—LOD is posterior-lateral and superior

Step 2: Contact sacrotuberous ligament—LOD isposterior-lateral and superior

Step 3: Contact lateral aspect of ilium—LOD isanterior-superior

Contact superior aspect of pubic bone—LOD isinferior

Contact inferior aspect of pubic bone—LOD issuperior

Table 1. Summary table of tests and adjustments AMCT Basic Scan Protocol*

Possibility 1: Raise legs to position 2; PD leg lengthens

Subluxation Test performed Adjustment: Contact + LOD

KneeMedial knee joint

Talus

Lateral knee joint

Cuboid

PelvisAS ilium

PI ilium

Symphysis pubis

Superior pubicbone

Inferior pubicbone

Pressure test medial collateral ligament lateral andinferior

Adjust when test for medial knee joint is positive

Pressure test lateral collateral ligament medial andinferior

Adjust when test for lateral knee joint is positive

Pressure test crest of ilium inferior-medial

Pressure test under sacrotuberous ligament posterior-superior and lateral

Instruct patient to squeeze knees together

When PD leg lengthens going to position 2

When PD shortens going to position 2

AMCT indicates Activator Methods chiropractic technique; PD, pelvic deficient side (short leg); LOD, line of drive; AS, anterosuperior (ilium); PI,posteroinferior (ilium).

*The AMCT basic scan protocol will be continued by next testing L5 and the rest of the vertebrae and shoulders as indicated.Source: Reprinted with permission from Fuhr AW, Green JR, Collaca TS, Keller TS, Activator Methods Chiropractic Technique, pp. 114–115, ©

1997, Mosby, Inc.

ological. The AAI evolved in response to current knowledgein both domains. Under the biomechanical model, issues suchas tissue compliance (stiffness), response to input force (im-pedance), and natural frequency resonance of the spine were

34 TOPICS IN CLINICAL CHIROPRACTIC/VOLUME 9, ISSUE 3, 2002

explored. In neurophysiological investigations, threshold fre-quencies and minimal forces required for stimulation of jointmechanoreceptors were investigated.

Two biomechanical publications emerged from the initial1985 NIH grant. This led to Fuhr’s first papers in the Journalof Manipulative and Physiological Therapeutics.24,25 The firststudy introduced a novel impedance-head–equipped spring-loaded AAI that caused measurable bone movement anddetectable electromyographic (EMG) response. This articleencouraged investigation of relative bone movement in re-sponse to light manipulative taps to the spine. One millimeterrelative translations and 0.5 degrees of rotation occurredduring the first 19 milliseconds. This was the first study toconfirm that an AAI thrust could induce motion in a bone,although the first research subject was a 40-pound dog. In thesecond article, the authors suggested that with further devel-opment piezoelectric accelerometers attached to an Activatorcould be a noninvasive tool to study dynamic, relative bonemovement.

The next research objective was to investigate the effects ofAAI adjustments on the human spine in vivo. This procedurerequired insertion of “Steinman pins” into the spinous pro-cesses of human subjects under local anesthesia. The pinswere placed into the spinous processes of the L4 and L5vertebrae of a live subject and Activator thrusts were made onthe spinous processes from T11 inferior to the L2 level. Thesethrusts exerted peak forces of approximately 72 newtons. Inresponse, the L4 and L5 vertebrae experienced an axialdisplacement, posteroanterior shear displacement, and rota-tional displacement at the L3-L4 spinal segment levels.26,27

Coupled spinal motion was clearly detected in more than justthe vertebra receiving a direct thrust. This study provided thefirst substantiated evidence in humans of vertebral displace-ment produced by an adjusting instrument.

Subsequent biomechanical research conducted with Kellerand others found, among other things, that when the instru-ment was equipped with a particular type of impedance head,it produced “a significant improvement in the frequencycontent” of the force delivered to the spine.27–31

Subsequent research guided further improvements in theforce-frequency spectrum of the AAI. That is, as force deliv-ery from the instrument was compared to known and discov-ered characteristics of the spine and nervous system, theinstrument itself was refined to match these characteristics.As an example, the Activator II adjusting instrument incorpo-rated an additional integral mass (about 45 g) attached to thestylus, which reduced momentum and thus better matched theresonance frequencies of the spine.

Later, it was found that by adding a preload control frame,impedance measurements were more reproducible. This meantpreload compression that would reduce force and changefrequency of the AAI springs were eliminated.26,30 Theseresults demonstrated an enhancement of the force-frequency

Table 2. Possibility 2: Leg stays short or becomesshorter upon flexion*

Raise legs to position 2—PD leg shortens

Subluxation Test Adjustment

Contact mammillaryprocess on L4 on sideopposite PD—LOD isanterior-superior

L4—subluxatedon sideopposite PD

PD leg goes shorton raising legs toposition 2

AMCT indicates Activator Methods chiropractic technique; PD, pel-vic deficiency.

*Continue the rest of the AMCT basic scan protocol by next testing L2and the rest of the vertebrae and shoulders as indicated.

Source: Reprinted with permission from Fuhr AW, Green JR, CollacaTS, Keller TS, Activator Methods Chiropractic Technique, pp. 114–115,© 1997, Mosby, Inc.

Table 3. Activator Methods basic scan protocol

Lower extremitiesMedial kneeLateral knee

Upper thoracic spineT6 and corresponding ribT4 and corresponding ribT l and first rib

Upper extremitiesMedial scapula

Lateral scapula

PelvisAS iliumPI iliumSymphysis pubis

Cervical spine and occiputC7, C5, C2, C1

Lumbar spineL5, L4, L2

ThoracicT12T8 and rib as indicated

Source: Reprinted with permission from Fuhr AW, Green JR, CollacaTS, Keller TS, Activator Methods Chiropractic Technique, p. 113, ©1997, Mosby, Inc.

Fuhr and Menke: Activator Methods’ Chiropractic Technique 35

spectrum, leading to development of the Activator III. Figure1 is a summary of the different characteristics and improve-ments in the three Activator models. Comparing the forcegraphs shows improvement in performance in the AAI over itsdevelopment. The force profile of the Activator II was en-hanced over the frequencies of 10 to 100 Hz in comparison tothe Activator I. The enhancement was increased by nearly twoorders of magnitude (1.0 E4 for Activator II versus 6.5 E2 forActivator I). Activator III in turn enhanced the frequencies of 2Hz to 100 Hz by yet another order of magnitude (1.0 E5 forActivator III versus 1.0 E4 for Activator II).

Certain biomechanical models suggest the spine responds—or resonates—most efficiently to specific frequency compo-nents of mechanical signals.27 One finding in Activator re-search was that the spine has a natural posterior to anteriorresonance frequency of between 30 and 50 Hz (cycles persecond). This means that forces applied at this frequency areactually transmitted and magnified along the entire spine.27,29

Slower rate spinal manipulative therapy (SMT) forces (1 to3 seconds) have not been found to produce neuromuscularreflex responses in the back musculature, regardless of whetherjoint cavitations were elicited.32–36 This is consistent with theAAI in its design to impart a consistent high loading rate, shortduration, multifrequency thrust that may not ordinarily beassociated with an audible joint cavitation.

In investigating the effect of the AAI on live subjects,27 theposterior mechanical behavior of the human thoracolumbarspine was found to be sensitive to mechanical frequency andshowed significant region-specific and gender differences. Inthe frequency range of 30 to 50 Hz, the lumbar spine of thissubject population was the least stiff and therefore had thegreatest mobility. The results of this study indicated thatdynamic spinal manipulative therapy procedures that induceimpulses at this frequency would produce more spinal motionfor a given force; given the muscle activity is kept to aminimum during thrust. Muscle action has the effect ofdampening force input by absorbing it. This can alter thefrequencies of the force reaching mechanoreceptors.37 In thisregard, spinal manipulative therapy procedures designed totarget the resonant frequency of the spine require less forceapplication. It appears that the frequency content of the AAIenergy impulse associated with mechanical stimulation dur-ing SMT has a direct bearing on the subsequent physiologicresponse of the body (see Figure 1 for a comparison of theforce frequency profiles of the AAIs).

Subsequently, the Activator instrument was used in a studymeasuring the stiffness of low back muscles in patients withand without chronic low back pain.38 Activator manipulativethrusts were delivered to several lumbosacral spinal land-marks and neuromuscular reflex responses were measured.Findings established significantly greater stiffness in lowback musculature on those with chronic low back pain. The

prototype of the Activator III used in this study confirmedearlier design improvements.

Several studies suggest patients suffering from back painrespond as effectively to the AAI as they do to hands-onlytherapy using higher force, low-loading rate thrusts.39,40 In-strument adjusting produces vibrations or oscillations in thespinal structures, whereas the latter is relatively localized orstatic in its effects, much the same as hands-only adjusting,which results in a joint cavitation and audible “pop” producedwithin the posterior spinal structures. The necessity for jointcavitation to produce therapeutic effects has been explored,29,36

and is still is a source of controversy within the chiropracticprofession.

Some evidence suggested that specific frequency compo-nents of mechanical signals27,41,42 might influence or enhancethe healing and remodeling of the musculoskeletal system(remodeling refers to the ability of biologic soft and hardtissues to alter their composition and structure in response tomechanical and systemic stimulation). Under this model,dynamic mechanical stimuli that match natural resonancecharacteristics of body tissues have a greater potential fortherapy than discrete static mechanical thrusts without reso-nant frequency content. Hence the rationale for low-forcedynamic chiropractic manipulations to affect the spine andparaspinal tissues. Hands-only manipulation to reduce jointfixation or produce changes in relative bone position may thusbe effective by matching a frequency profile required fortherapeutic changes. Thus “both magnitude and frequency ofhands-only and mechanical thrusting manipulations are criti-cal elements in determining therapeutic effects.”26,27,43

Solinger further addressed the issue of resonance by com-paring the spine to a damped harmonic oscillator model. Henoted that hands-only and Activator spinal adjustments cre-ated the same oscillating frequencies of the spine, though thehands-only adjustments produced higher amplitudes.29,43 Thusit may be inferred that both instrument and hands-only adjust-ments are similar in efficacy if oscillating frequencies are theimportant end result. Thus one might surmise that both treat-ments are equally the same in clinical application. This hasbeen supported in some studies.40,44

Neurophysiological dynamics

Chiropractic adjustments delivered by AAI may “normal-ize” articular afferent input to the central nervous system withsubsequent recovery of muscle tone, joint mobility, and sym-pathetic activity.45 Gillette46 hypothesized that a chiropracticlumbar thrust would produce sufficient force to coactivate allof the mechanically sensitive receptor types he reviewed,because it generates over 40 newtons of force. Adjustmentsmade with the AAI are thought to accomplish the same task,because they have been reported to produce well over 40 N offorce.47 Brodeur48 proposed that the AAI has the capacity to

36 TOPICS IN CLINICAL CHIROPRACTIC/VOLUME 9, ISSUE 3, 2002

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Fuhr and Menke: Activator Methods’ Chiropractic Technique 37

coactivate type III, high-threshold mechanoreceptors. Morerecent research has shown that both types III and IV receptorsin diarthrodial joints, as well as type II in paravertebralmuscles and tendons are responsive to vertebral displace-ment.48 It has also been shown that muscle activation iselicited by fast thrusts, not slow ones—regardless of jointcavitation sounds.33,50 Figure 2 shows the range of frequenciesgenerated from the AAI III as compared to the frequencythreshold ranges of three major mechanoreceptors, the Merkeldiscs, and Meissner and Pacinian corpuscles.51 AAI III im-proves force magnitude across the frequency thresholds, thusimproving the possibility of neurological response of theadjustment.

Speed of the thrust, however, was found to be important ineliciting neural responses—regardless of the presence ofcavitation. In a series of thoracic spinal thrusts producingaudible joint cavitation, only fast thrusts producedelectromyelogram activity in paraspinal musculature.50 Herzogconfirmed that cavitation alone is not responsible for backmuscle reflex responses. In his study, magnitude of appliedforce was found to have little importance in bringing about aresponse in muscle reflexes.36

Increases in neurophysiological response to various direc-tions of force vectors applied to mechanoreceptors are knownas “receptive fields.”49,52–54 One study using the controlledforce vectors of the AAI lent support to the importance ofthrust direction, or line of drive. In this study, mixed spinal

nerve discharge measured in response to an Activator adjust-ment delivered directly on lumbosacral vertebrae.55 A patientvolunteer was prepared for back surgery. An incision wasmade over the L3-S2 midline and the S1 nerve root wasmonitored at the dorsal root ganglion on the right side.

In phase I, an internal Activator adjustment was madedirectly to the mammillary process of L5. In the second phase,an Activator thrust was made to the skin and tissue overlyingthe spine at various locations. In both cases, a mixed nerve rootresponse was measured. The research yielded several impor-tant findings. First, the Activator impulse traversed the softtissue and elicited a mixed nerve response similar to directthrust on bone. Second, sensitivity to direction of force wassupported because an anterior superior line of drive increasedmixed nerve response by as much as three times, supportingearlier research by Pickar and McLain.54 Surprisingly externalspinal manipulative thrusts caused a greater range of neuralresponse in comparison to internal ones, especially when theline of drive was anterior superior in direction (Table 4).

Herzog and colleagues36 investigated forces delivered tothe spine with manual, high-velocity, low-amplitude (HVLA)adjustments. Manual manipulation produced relative move-ments in the target vertebra of 1 mm translation and 1 degreeof rotation. This was comparable to vertebral movementeffected by AAI. Herzog’s group found that although totalforces are greater with manual adjustments—because of thelarger contact area—local peak or specific forces on the target

Fig. 2. Results of fast Fourier transform of Activator III force impulse

38 TOPICS IN CLINICAL CHIROPRACTIC/VOLUME 9, ISSUE 3, 2002

vertebrae itself are the same for Activator and hands onlymanipulation. They also found that hands-only manipulationdelivers the peak force over 150 to 200 milliseconds. Thiscompares to a delivery of about 3 milliseconds for the AAI.

In measuring neuromuscular responses of back muscleswith surface EMGs in response to Activator treatment, Symonsand colleagues34 found about 68% of the thrusts resulted in adetectable response. Specifically, the cervical spine respondedto 50% of the thrusts; thoracic spine, 59%; lumbar spine, 83%;and sacroiliac joints, 94%. Colloca and Keller38 reportedneuromuscular responses were elicited in 95% of the 20 lowback patients from AAI adjustments. In this study, surfaceelectromyographic electrodes monitored lumbar paraspinalmuscles as they responded consistently to a modified Activa-tor adjustment of specific vertebral segments. Another studyof back pain patients found significant but temporary in-creases in trunk muscle strength as a result of AAI lumbosac-ral adjustments.56 Neurological effects via vertebral displace-ment may extend beyond somatic pain and dysfunction,according to Bolton.49 He noted some types of vertebral

Table 5. A comparison of forces and thrust duration of Activator Adjusting Instrument III versus manual manipulation

Spinal region

Mode of adjustment Cervical Thoracic Lumbosacral/sacroiliac

(AAI III) Preload force 22 N 22 N 22 N

Peak impulse force 121 N 123 to 150 N 125 to 150 N

Impulse duration 3 to 5 ms 3 to 5 ms 3 to 5 ms

Manual manipulation* PreloadPreload force 0 to 50 N 139 ± 46 N 328 ± 78 N

Peak impulse force 40 to 120 N 88 ± 78 N 399 ± 119 N

Impulse duration 30 to 129 ms 150 ± 77 ms 150 ± 77 ms

Source: Data from Pickar JG. Neurophysiological effects of spinal manipulation. The Spine Journal, in press.

Table 4. Average mixed-nerve root responses (mV) to spinal manipulative thrusts delivered internally and externallyat different segmental levels and with differing force vectors

L5 Ant LOD L5-Ant-sup LOD S1-Ant-inf LOD

Internal spinal manipulative thrusts 500–1,200 1,200–2,600 200–900External spinal manipulative thrusts 1,200 800–3,500 900

LOD indicates line of drive; Ant, anterior; Sup, superior; Inf, inferior.Source: Reprinted with permission from Colloca CJ, Keller TS, Gunzberg R, et al., Neurophysiologic response to intraoperative lumbosacral spinal

manipulation, J Manipulative Physical Ther, Vol. 23, No. 7, p. 453, © 2000, Mosby, Inc.

displacements modulate heart rate, blood pressure, and elec-trical activity in renal and adrenal nerves and in gastrointesti-nal muscles.

There are at least two components to the chiropracticadjustment: biomechanical and neurophysiological. Two majorapproaches for adjusting the spine are instrument and hands-only, and they appear to be effective via clinical experience.Some comparative clinical research is discussed in the nextsection. However, in terms of application to the body, hands-only and AAI are distinctly different. Table 5 compares thetwo approaches in terms of preload forces, adjustive forces,and thrust duration.57

CLINICAL RESEARCH

From a biomechanical perspective, AAI thrusts appearequivalent to hands-only. However, is the AAI adjustmenttherapeutically equivalent to manual adjusting in terms ofclinical outcomes? Two studies have attempted to address thisquestion by means of randomized clinical trials. Gemmel and

Fuhr and Menke: Activator Methods’ Chiropractic Technique 39

Jacobson39 compared a single AMCT to a single manualadjustment in 30 acute low back pain patients. They found nosignificant differences in pain reduction between the twomethods, though no control group was used to compare tonatural history. Wood and colleagues40 compared Activatoradjusting to manual manipulation in the treatment of cervicalspine dysfunction. The 30 patients who had neck pain andrestricted range of motion were randomly assigned into twogroups, one treated with an Activator II instrument, the otherby manual HVLA diversified adjusting. At the end of thetreatment period—a maximum of eight treatments or untilasymptomatic status was reached—both groups experienceda significant decrease in symptoms. The AMCT group actu-ally showed a greater improvement in cervical range ofmotion from pre- to posttreatment, and from posttreatment to1-month follow-up, as compared to the diversified group.Also, the AMCT group experienced range of motion improve-ment in all directions, whereas the diversified group experi-enced improvement in left rotation. However, because neitherstudy had control groups, and thus no comparison to naturalhistory, interpretation as to either treatment efficacy must bemade with caution.

Yates and colleagues58 tested whether spinal adjustments toT1 to T5 subluxations could have an effect upon bloodpressure and anxiety. Twenty-one subjects were randomlyassigned to three treatment groups: adjustment (by AAI),placebo (AAI set to zero force), and baseline (no treatment)control. Subjects’ systolic and diastolic blood pressures andanxiety were measured. All groups were equivalent in theirpretreatment scores and stable in blood pressure measure-ments from baseline to pretreatment evaluation. Analysis ofchange scores from pre- to posttreatment showed significantdifferences between the three groups with respect to decreasesin systolic, diastolic, and state anxiety. Specifically, systolicand diastolic pressures were reduced only in the Activatortreatment group. State anxiety was reduced more in theActivator treatment group, but this decrease was not signifi-cantly different from the control.

In another study,59 10 consecutive new patients with ahistory of neck injury and 9 asymptomatic volunteer controlswere subjected to a regime of mechanical force manuallyassisted short lever chiropractic adjustments from an AAI.Although acute, 4 of the 10 also received interferential elec-trical stimulation current to the rhomboids. Visual analogscales measured pain whereas dysfunction was measured byfinite helical axis parameters (FHAP) that closely mirroredthe clinical condition of patients. The FHAP patterns in thetreatment group differed markedly from FHAP patterns ob-served in the asymptomatic control group for all but one of thepatients. After 6 weeks, the mean pain scores decreased andfunction increased significantly over the course of treatmentin comparison to the control group (p < 0.001 for pain and p

< 0.001 for FHAP). After 1 year, 7 of the 10 treated patientsnoted stability of their symptoms at or near the level reportedimmediately after the 6-week treatment period.

In a small randomized trial, 14 patients with neck painrandomly were assigned to either manual therapy or Activatortreatment. Both groups saw an appreciable but not statisticallysignificant reduction in pain and increase in range of motion.However, there were no differences between the Activatorand hands-only adjusting in terms of pain and function out-comes.44 Yet another case series of 10 patients under Activatormanagement had a significant reduction of pain in a chronicsacroiliac joint syndrome. Under this study, a 6-week regimenof AAI adjustments reduced pain from mean baseline valuesof 25 to 12, and average disability scores diminished from28% to 13% (both p < 0.05).60

Randomized, placebo-controlled clinical trials should have,among other attributes, a comparison treatment similar to thetreatment intervention except for an “active ingredient” be-lieved to be responsible for clinical change. In the case ofspinal manipulation or adjustment, this is the thrust deliveredto the spinal site indicated by the chiropractic system ofanalysis. With manual adjustment studies, sham placebos aredifficult to design because they are often not similar enoughto the actual manipulation. The AAI, on the other hand, lendsitself easily to controlled clinical trials because of its ability toreduce force transmission to zero while giving the appearanceand sound of an actual Activator adjustment. Several investi-gators have highlighted the usefulness of the Activator inclinical research.56,61 Chiropractic for the first time has awidely used clinical intervention readily adaptable to theresearch setting.

Case series and case studies

Two types of observational studies are of special interest toclinicians: individual case studies and clinical series. Theformer examines a single patient in depth; the latter is anaccumulation of individual patients measured on the sameoutcomes. Case studies have many limitations in validity, butare useful for illustrating clinical phenomena and for introduc-ing research hypotheses.

Polkinghorn and Colloca62 reported an apparently success-ful treatment for unremitting coccygodynia of 3 weeks dura-tion in a 29-year-old female. Isolation tests of the sacrococ-cygeal region indicated the coccyx had subluxated laterally.Significant relief was appreciated after the very first Activatortreatment: an adjustment near the base of the coccyx per-formed as indicated by AMCT analysis protocol prescribedcontact and specified line of drive. The patient was treateduntil the leg checks indicated her problem had resolved.

Case studies have suggested successful resolution of lumbardisc herniation signs and symptoms63 and cervical disc protru-sion signs and symptoms64 by AMCT management. In the latter

40 TOPICS IN CLINICAL CHIROPRACTIC/VOLUME 9, ISSUE 3, 2002

Table 6. Summary table of Activator prospective grouptrials

Colloca CJ, Keller TS. Electromyographic reflex responses tomechanical force, manually assisted spinal manipulativetherapy. Spine. 2001;26(10):1117–1124.

Colloca CJ, Keller TS. Stiffness and neuromuscular reflexresponse of the human spine to posteroanterior manipula-tive thrusts on patients with low back pain. J Manip PhysTher. 2001;24(8):489–500.

Gemmell HA, Jacobson BH. The immediate effect of Activa-tor vs. meric adjustment on acute low back pain: arandomized controlled trial. J Manip Phys Ther.1995;18(7):453–456.

Keller TS, Colloca CJ, Fuhr AW. In vivo transient vibrationassessment of the normal human thoracolumbar spine. JManip Phys Ther. 2000;23(8):521–530.

Keller TS, Colloca CJ. Mechanical force spinal manipulationincreases trunk muscle strength assessed by electromyogra-phy: a comparative clinical trial. J Manip Phys Ther.2000;23(9):585–595.

Nathan M, Keller TS. Measurement and analysis of the in vivoposteroanterior impulse response of the human thora-columbar spine: a feasibility study. J Manip Phys Ther.1994;17(7):431–441.

Osterbauer PJ, DeBoer KF, Widmaier RS, et al. Treatment andbiomechanical assessment of patients with chronicsacroiliac joint syndrome. J Manip Phys Ther.1993;16(2):82–90.

Osterbauer PJ, Derickson KL, Peles JD, et al. Three-dimen-sional head kinematics and clinical outcome of patientswith neck injury treated with spinal manipulative therapy:a pilot study. J Manip Phys Ther. 1992;15(8):501–511.

Wood TG, Colloca CJ, Matthews R. A pilot randomizedclinical trial on the relative effect of instrumental versusmanual thrust manipulation in the treatment of cervicalspine dysfunction. J Manip Phys Ther. 2001;24(4):260–271.

Yates RG, Lamping NL, Abram C, et al. The effects ofchiropractic treatment on blood pressure and anxiety: arandomized, controlled trial. J Manip Phys Ther.1988;11(6):484–488.

case, a 42-year-old woman with cervical disk protrusion unre-sponsive to manual manipulation showed a “favorable” re-sponse during the first week of AMCT management. All signsand symptoms resolved by the end of 3.5 months.

Other case studies have shown promise for AMCT analy-sis. The treatment of a torn medial meniscus,65 plantar fasciitisattributed to posterior calcaneus subluxation,66 otitis mediawhen related to upper cervical segmental dysfunction,67 Bell’spalsy,68 adhesive capsulitis (frozen shoulder),69 postsurgicalneck syndrome,70 chest pain,71 acute torticollis,72 and sciaticneuropathy and low back pain associated with lumbar discherniation.73,74

SAFETY

When any novel health care device is employed, safety is aprimary concern and must be satisfactorily established. Thusthe first NIH grant ever awarded for chiropractic-relatedresearch investigated the safety and efficacy of spinal manipu-lation by use of an adjusting instrument. The goal of theresearch was to investigate the safety of the AAI, an adjustinginstrument with an internal hammer hitting a stylus. Articlesproduced from the grant found that the spring driving thishammer could not have produced more than 0.3 J of potentialenergy to be effective but safe. In testing, the AAI was foundquick to complete a thrust, but yet never achieved greatvelocity because of the short distance traveled.24,25

In a survey of chiropractors in Denmark on the safety ofcervical adjustments, upper cervical rotational adjustmentsappeared to pose a greater risk for cerebrovascular accidentsor cerebrovascular incidents.75 The authors suggested thatAAI adjusting was not involved in any adverse incidents in theDanish chiropractic patients. It was inferred that AMCT is anacceptable form of chiropractic treatment in cases in whichrotation is contraindicated.76 This idea was lent some supportby Kawchuk and Herzog,77 in which five chiropractic treat-ment types were compared. Activator adjusting exhibited alow peak force and the lowest thrust duration.

As with any intervention with therapeutic potential, risksexist. Nykoliation and Mierau78 published a review of threealleged injury cases in Canada possibly associated with me-chanical adjusting devices, though neither the AAI nor AMCTanalysis was identified. The single reported case of stroke wassubsequent to a motor vehicle accident with headaches andneck pain and included multiple modalities of cervical trac-tion, diathermy, and conventional spinal manipulation inpatient management. With the accident and other treatments,a clear causal link to mechanical adjusting was not estab-lished. Two other cases included a patient with thoracic paintreated by a mechanical adjusting device accompanied by“twisting motions” inconsistent with AMCT protocols, andanother with neck pain, headaches, and right arm paraesthesias

unresponsive to a mechanical adjusting instrument, but whosubsequently responded to manual adjusting.

CONCLUSION

Instrument adjusting has a long history preceding the chi-ropractic profession. The AMCT was initially developed as anoutgrowth of the “thumb-thrust” technique, a type of lowforce adjustment. Later the technique incorporated and modi-fied Derifield leg length checks as a primary clinical indicator.AMCT has been a leader in chiropractic research with at least54 studies related to the biomechanics, neurophysiology,

Fuhr and Menke: Activator Methods’ Chiropractic Technique 41

Table 7. Summary table of Activator Methods casestudies

Frach JP, Osterbauer PJ. Chiropractic treatment of Bell’s palsyby Activator instrument adjusting and high voltageelectrotherapy: a report of two cases. J Manip Phys Ther.1992;15(9):596–598.

Henningham M. Activator adjusting for acute torticollis.Chiropr J Austr. 1982;2:13–14.

Phillips NJ. Vertebral subluxation and otitis media: a casestudy. Chiropractic. 1992;8(2):38–39.

Polkinghorn BS. Chiropractic treatment of frozen shoulder(adhesive capsulitis) utilizing mechanical force, manuallyassisted short lever adjusting procedures. J Manip PhysTher. 1995;18(2):105–115.

Polkinghorn BS. Conservative treatment of torn medialmeniscus via mechanical force, manually assisted, shortlever chiropractic adjusting procedures. J Manip Phys Ther.1994;17:474–484.

Polkinghorn BS. Grand rounds discussion: patient with acutelow back pain. Chiropr Tech. 1999;11(1):1–32.

Polkinghorn BS. Instrumental chiropractic treatment of frozenshoulder associated with mixed metastatic carcinoma.Chiropr Tech. 1995;7(3):98–102.

Polkinghorn BS. Posterior calcaneal subluxation; its impor-tance in conservative treatment of heel spur syndrome(plantar fasciitis) via instrumentized chiropractic adjustingprocedures. J Chiropr Sports Med. 1995;9(2):44–51.

Polkinghorn BS. Treatment of cervical disc protrusion viainstrumental chiropractic adjustment. J Manip Phys Ther.1998;21(2):114–121.

Polkinghorn BS, Colloca CJ. Chiropractic treatment ofcoccygodynia via external instrumental adjusting proce-dures utilizing Activator Methods Chiropractic Technique. JManip Phys Ther. 1999;22(6):411–416.

Polkinghorn BS, Colloca CJ. Chiropractic treatment ofpostsurgical neck syndrome utilizing mechanical force,manually-assisted short lever spinal adjustments. J ManipPhys Ther. 2001;24(9):589–595.

Polkinghorn BS, Colloca CJ. Treatment of symptomaticlumbar disc herniation utilizing Activator MethodsChiropractic Technique. J Manip Phys Ther.1998;21(3):187–196.

Richards GL, Thompson JS, Osterbauer PJ, Fuhr AW. Lowforce chiropractic care of two patients with sciatic neuropa-thy and lumbar disc herniation. Am J Chiropr Med.1990;3(1):25–32.

Yurkiw D, Mior S. Comparison of two chiropractic tech-niques on pain and lateral flexion in neck pain patients: apilot study. Chiropr Tech. 1996;8:155–162.

clinical assessment reliability, and clinical outcomes (Tables6 and 7). AMCT also has led in organizing and codifying anapproach to chiropractic care by publishing a comprehensivetextbook on the subject.79

The first Activator training seminar was offered in 1969,and has grown to 32 seminars per year, with certification ofmore than 1,500 chiropractors in Activator Methods basictechnique and 600 chiropractors in the advanced technique.Recent data from the National Board of Chiropractic Examin-ers find the Activator technique the most used name-brandtechnique in use by chiropractors, with 63% of chiropractorsclaiming use on about 22% of their patients in 1998. AMCTappears to be among the fastest growing, with a 24% growthin chiropractic usage between 1991 and 1998.1

Activator Methods chiropractic technique offers spinal andextremity adjusting with clear training and competency stan-dards and objectives. In studies, the AAI and AMCT resultsare similar to hands-only adjusting procedures. Force-deliv-ery characteristics and lines-of-drive are along planes of jointwithout segment rotation, which may reduce risk of unin-tended and iatrogenic effects of spinal manipulative therapyfor high-risk patients.

As with all chiropractic science and technique, the AMCTmust address issues of clinical efficacy and cost-benefit toaddress the demands of quality health care in the future andhelp define chiropractic’s role in the future of health caredelivery.

REFERENCES

1. Christensen MG, Kerkoff D, Kollasch MW, Cohn L. Job Analysis ofChiropractic: A Project Report, Survey Analysis and Summary of thePractice of Chiropractic Within the United States. Greeley, CO:National Board of Chiropractic Examiners, 2000.

2. Pedersen P: A survey of chiropractic practice in Europe. Eur JChiropr. 1994;425(1):3–28.

3. Gleberzon BJ. Chiropractic “name techniques”: a review of theliterature. J Can Chiropr Assoc. 2001;45(2):86–99.

4. Gleberzon BJ. Name techniques in Canada: current trends inutilization rates and recommendations for their inclusion in theCanadian Memorial Chiropractic College. J Can Chiropr Assoc.2000;44(3):157–168.

5. Keating JC: Mechanical force, manually assisted short lever chiro-practic adjustment [letter to the editor]. J Manip Physiol Ther.1993;16:55.

6. Dorn WH: Indian lore: crude use of chiropractic fundamentalscenturies ago. Chiropr J (NCA). 1935;4:17–18.

7. Zarbuck MV, Hayes MB: Following D.D. Palmer to the West Coast:the Pasadena connection, 1902. Chiropr Hist. 1990;10:17–22.

8. Kell PM: A historical review of the instruments of chiropractic shortlever manipulation. San Diego, CA: unpublished data, 1991.

9. Smallie P. Introduction to Ratledge Files and Ratledge Manuscript.Stockton, CA: World-Wide Books, 1990.

10. Osterbauer PJ, Fuhr AW. The current status of Activator MethodsChiropractic Technique, theory, and training. Chiropr Tech.1990;2(4):168–175.

11. Richards DR. The Activator story: development of a new concept inchiropractic. Chiropr J Austr. 1994;24(1):28–32.

12. Cooperstein R. Activator methods chiropractic technique. ChiroprTech. 1997;9(3);108–114.

42 TOPICS IN CLINICAL CHIROPRACTIC/VOLUME 9, ISSUE 3, 2002

13. Kaminski M, Boal R, Gillette R, et al. A model for the evaluation ofchiropractic methods. J Manip Physiol Ther. 1987;10(2):61–64.

14. Fuhr AW, Osterbauer PJ. Interexaminer reliability of relative leg-length evaluations in the prone, extended position. Chiropr Tech.1989;1(1):13–18.

15. DeWitt JK, Osterbauer PJ, Stelmach GE, Fuhr AW. Optometricmeasurement of changes in leg length inequality resulting fromisolation tests. J Manip Physiol Ther. 1994;17(8):530–538.

16. Youngquist MW, Fuhr AW, Osterbauer PJ. Interexaminer reliabilityof an isolation test for the identification of upper cervical sublux-ation. J Manip Physiol Ther. 1989;12(2):93–97.

17. Nguyen HT, Resnick DN, Caldwell SG, et al. Interexaminer reliabil-ity of Activator methods’ relative leg-length evaluation in the proneextended position. J Manip Physiol Ther. 1999;22(9):565–569.

18. Jansen RD, Cooperstein R. Measurement of soft tissue strain inresponse to consecutively increased compressive and distractiveloads on a friction-based test bed. J Manip Physiol Ther.1998;21(1):19–26.

19. Shambaugh P, Solafani L, Fanselow D. Reliability of the Derefield-Thompson test for leg-length inequality, and use of the test todemonstrate cervical adjusting efficacy. J Manip Physiol Ther.1988;11(5):396–399.

20. Plaugher G: Textbook of Clinical Chiropractic. Baltimore: William& Wilkins, 1993.

21. Osterbauer P, Fuhr AW, Keller TS: Description and Analysis ofActivator Methods Chiropractic Technique, Advances in Chiro-practic, Vol 2, St. Louis: Mosby, 1995, 471–520.

22. Haldeman S, Chapman-Smith D, Petersen DM Jr, eds: Guidelinesfor chiropractic quality assurance and practice parameters: pro-ceedings of the Mercy Center Consensus Conference. Gaithersburg,MD: Aspen Publishers Inc., 1993.

23. Henderson D, Chapman-Smith D, Mior S, Vernon H. Clinicalguidelines for chiropractic practice in Canada. J Can Chiropr Assoc.1994;38(1, suppl):1–203.

24. Fuhr AW, Smith DB: Accuracy of piezoelectric accelerometersmeasuring displacement of a spinal adjusting instrument. J ManipPhysiol Ther. 1986;9:15–21.

25. Smith DB, Fuhr AW, Davis BP. Skin accelerometer displacementand relative bone movement of adjacent vertebrae in response tochiropractic percussion thrusts. J Manip Physiol Ther. 1989;12:26–37.

26. Keller TS. Engineering—in vivo transient vibration analysis of thenormal human spine. In Fuhr AW, Green JR, Collaca CJ, Keller TS,eds. Activator Methods Chiropractic Technique. St. Louis: Mosby,1997.

27. Keller TS, Colloca CJ, Fuhr AW. In vivo transient vibration assess-ment of the normal human thoracolumbar spine. J Manip PhysiolTher. 2000;23(8):521–530.

28. Keller TS, Lehneman JB. Dependence of the delivered force on theforce setting on the Activator adjusting instrument. Tech Report,Phoenix, AZ: Activator Methods, 1994.

29. Solinger AB. Oscillations of the vertebrae in spinal manipulativetherapy. J Manip Physiol Ther. 1996;19(4):238–243.

30. Keller TS, Colloca CJ, Fuhr AW. Validation of the force andfrequency characteristics of the Activator™ adjusting instrument:effectiveness as a mechanical impedance measurement tool. JManip Physiol Ther. 1999;22(2):75–86.

31. Herzog W. Mechanical and physiological responses to spinalmanipulative treatments. J Neuromusc Sys. 1995;3(1):1–9.

32. Herzog W. Mechanical, physiologic, and neuromuscular consider-ations of chiropractic treatments. In: Lawrence DJ, Cassidy JD,McGregor M, et al., eds. Advances in Chiropractic. St. Louis:Mosby-Year Book, 1996, 269–285.

33. Herzog W. On sounds and reflexes. J Manip Physiol Ther.1996;19(3):216–218.

34. Symons BP, Herzog W, Leonard T, Nguyen H. Reflex responsesassociated with Activator treatment. J Manip Physiol Ther.2000;23(3):155–159.

35. Colloca CJ, Keller TS. Electromyographic reflex responses to me-chanical force, manually assisted spinal manipulative therapy.Spine. 2001;26(10):1117–1124.

36. Herzog W, Kats M, Symons B. The effective forces transmitted byhigh-speed, low-amplitude thoracic manipulation. Spine.2001;26(19)2105–2111.

37. Triano JJ. The mechanics of spinal manipulation. In Herzog W, ed.Clinical Biomechanics of Spinal Manipulation. New York: ChurchillLivingstone, 2000, 92–190.

38. Colloca CJ, Keller TS. Stiffness and neuromuscular reflex responseof the human spine to posteroanterior manipulative thrusts onpatients with low back pain. J Manip Physiol Ther. 2001;24(8):489–500.

39. Gemmell HA, Jaconson BH. The immediate effect of Activator™versus meric adjustment on acute low back pain: a randomizedcontrolled trial. J Manip Physiol Ther. 1995;18(7):453–456.[AQ13]

40. Wood TG, Colloca CJ, Matthews R. A pilot randomized clinical trialon the relative effect of instrumental (MFMA) versus manual (HVLA)manipulation in the treatment of cervical spine dysfunction. JManip Physiol Ther. 2001;24(4):260–271.

41. Rubin CT, Lanyon LE. Osteoregulatory nature of mechanical stimuli:function as a determinant for adaptive remodeling in bone. J OrthopRes. 1987;5:300–310.

42. Hansson TH, Keller TS. Osteoporosis of the spine. In Wiesel S,Weinstein S, Herkowitz H, et al., eds: The Lumbar Spine, ed 2. NewYork: 1996, WB Saunders, 1996.

43. Solinger AB. Theory of small vertebral motions: an analytical modelcompared to data. Clin Biomechanics. 2000;15(2):87–94.

44. Yurkiw D, Mior S. Comparison of two chiropractic techniques onpain and lateral flexion in neck pain patients: a pilot study. ChiroprTech. 1996;8(4):155–161.

45. Henderson CNR. Three neurophysiological theories on the chiro-practic subluxation. In Gatterman MI, ed: Foundations of Chiro-practic Subluxation, St. Louis: Mosby, 1995.

46. Gillette RG. A speculative argument for the coactivation of diversesomatic receptor populations by forceful chiropractic adjustments.Manual Med. 1987;3(1):1–14.

47. Nathan M, Keller TS. Measurement and analysis of the in vivoposteroanterior impulse response of the human thoracolumbar spine:a feasibility study. J Manip Physiol Ther. 1994;17(7):431–441.

48. Brodeur R: The audible release associated with joint manipulation.J Manip Physiol Ther. 1995;18:155–164.

49. Bolton PS. Reflex effects of vertebral subluxations: the peripheralnervous system. An update. J Manip Physiol Ther. 2000;23(2):101–103.

50. Herzog W, Conway DC, Zhang YT, et al. Reflex responses associ-ated with manipulation treatments on the thoracic spine: a pilotstudy. J Manip Physiol Ther. 1995;18(4):233–366.

51. Gardner E., Martin JH, Jesell TM. The bodily senses. In Kandel ER,Schwartz JH, Jessell TM, eds. Principles of Neural Science. NewYork: McGraw-Hill, 2000, 438.

52. McLain RF. Mechanoreceptor endings in human cervical facetjoints. Spine. 1994;19:495–501.

53. McLain RF, Pickar JG. Mechanoreceptor endings in human tho-racic and lumbar facet joints. Spine. 1998;23(2):168–173.

54. Pickar JG, McLain RF. Responses of mechanosensitive afferents tomanipulation of the lumbar facet in the cat. Spine. 1995;20(22):2379–2385.

Fuhr and Menke: Activator Methods’ Chiropractic Technique 43

55. Colloca CJ, Keller TS, Gunzburg R, et al. Neurophysiologic re-sponse to intraoperative lumbosacral spinal manipulation. J ManipPhysiol Ther. 2000;23(7):447–457.

56. Keller TS, Colloca CJ. Mechanical force spinal manipulation in-creases trunk muscle strength assessed by electromyography: acomparative clinical trial. J Manip Physiol Ther. 2000;23(9):585–595.

57. Pickar JG. Neurophysiological effects of spinal manipulation. TheSpine Journal. 2002. In press.

58. Yates RG, Lamping DL, Abram NL, Wright C. Effects of chiropractictreatment on blood pressure and anxiety: a randomized, controlledtrial. J Manip Physiol Ther. 1988;11(6):484–488.

59. Osterbauer PJ, Derickson KL, Peles JD, et al. Three-dimensionalhead kinematics and clinical outcome of patients with neck injurytreated with spinal manipulative therapy: a pilot study. J ManipPhysiol Ther. 1992;15(8):501–511.

60. Osterbauer PJ, DeBoer KF, Widmaier RS, et al. Treatment andbiomechanical assessment of patients with chronic sacroiliac jointsyndrome. J Manip Physiol Ther. 1993;16(2):82–90.

61. Hawk C, Azad A, Phongphua C, Long CR. Preliminary study of theeffects of a placebo chiropractic treatment with sham adjustments.J Manip Physiol Ther. 1999;22(7):436–443.

62. Polkinghorn BS, Colloca CJ. Chiropractic treatment of coccygo-dynia via instrument adjusting procedures using Activator methodschiropractic technique. J Manip Physiol Ther. 1999;22(6):411–416.

63. Polkinghorn BS, Colloca CJ. Treatment of symptomatic lumbar discherniation using Activator methods chiropractic technique. J ManipPhysiol Ther. 1998;21(3):187–196.

64. Polkinghorn BS. Treatment of cervical disc protrusion via instru-mental chiropractic adjustment. J Manip Physiol Ther.1998;21(2):114–21.

65. Polkinghorn BS. Conservative treatment of torn medial meniscusvia mechanical force, manually assisted, short lever chiropracticadjusting procedures. J Manip Physiol Ther. 1994;17:474–484.

66. Polkinghorn BS. Posterior calcaneal subluxation; its importance inconservative treatment of heel spur syndrome (plantar fasciitis) viainstrumentized chiropractic adjusting procedures. J Chiropr SportsMed. 1995;9(2):44–51.

67. Phillips NJ. Vertebral subluxation and otitis media: a case study.Chiropractic. 1992;8(2):38–39.

68. Frach JP, Osterbauer PJ. Chiropractic treatment of Bell’s palsy byActivator instrument adjusting and high voltage electrotherapy: areport of two cases. J Manip Physiol Ther. 1992;15(9):596–598.

69. Polkinghorn BS. Chiropractic treatment of frozen shoulder (adhe-sive capsulitis) utilizing mechanical force, manually assisted shortlever adjusting procedures. J Manip Physiol Ther. 1995;18(2):105–115.

70. Polkinghorn BS, Colloca CJ. Chiropractic treatment of postsurgicalneck syndrome utilizing mechanical force, manually-assisted shortlever spinal adjustments. J Manip Physiol Ther. 2001;24(9):589–595.

71. Polkinghorn BS, Colloca CJ. Chiropractic management of chestpain utilizing mechanical force, manually assisted short leveradjusting procedures. J Manip Physiol Ther. 2002; In press.

72. Henningham M. Activator adjusting for acute torticollis. Chiropr JAustr. 1982;2:13–14.

73. Polkinghorn BS. Grand rounds discussion: patient with acute lowback pain. Chiropr Tech. 1999;11(1):1–32.

74. Richards GL, Thompson JS, Osterbauer PJ, Fuhr AW. Low forcechiropractic care of two patients with sciatic neuropathy andlumbar disc herniation. Am J Chiropr Med. 1990;3(1):25–32.

75. Klougart N, Leboeuf-Yde C, Rasmussen LR. Safety in chiropracticpractice. Part II: treatment to the upper neck and the rate ofcerebrovascular incidents. J Manip Physiol Ther. 1996;19(9):563–569.

76. Colloca CJ, Fuhr AW. Safety in chiropractic practice. Part II:treatment to the upper neck and the rate of cerebrovascularincidents [letter]. J Manip Physiol Ther. 1997;20(8):567–568.

77. Kawchuk GN, Herzog W. Biomechanical characterization (finger-printing) of five novel methods of cervical spine manipulation. JManip Physiol Ther. 1993;16:573–577.

78. Nykoliation J, Mierau D. Adverse effects potentially associated withthe use of mechanical adjusting devices: a report of three cases. JCan Chiropr Assoc. 1999;43(3):161–167.

79. Fuhr AW, Green JR, Collaca CJ, Keller TS. Activator MethodsChiropractic Technique. St. Louis: Mosby, 1997.